1. Field of the Invention
The present invention relates to a magnetron which is applied to a microwave oven or the like, and more particularly, it relates to a magnetron which has a cathode provided with an improved end shield.
2. Description of the Prior Art
FIG. 1 is a typical diagram schematically showing the structure of a microwave oven, for example, to which a magnetron is applied. Referring to FIG. 1, a microwave oven 1000 has a magnetron 100, a driving power source 200 for driving the magnetron 100 and a waveguide 300. The entire microwave oven 1000 is covered with a microwave oven cover 400. Microwaves oscillated from the magnetron 100 are guided into an internal space 500 of the microwave oven 1000 through the waveguide 300. Food 700 placed on a pan 600 is heated and cooked by such microwaves.
FIG. 2A is a partially fragmented front elevational view showing the structure of a conventional magnetron which is disclosed in Japanese Patent Publication Gazette No. 45340/1986, for example. FIG. 2B is a partial sectional view taken along the line IIB--IIB in FIG. 2A. FIG. 2C is a partial sectional view taken along the line IIC--IIC in FIG. 2B. The structure of such a typical conventional magnetron is now described with reference to these figures.
Referring to FIGS. 2A to 2C, a magnetron 100 is provided with a cathode 3 in its central portion. The cathode 3 has a filament 5 (see FIG. 2c), which emits electrons. A plurality of panel-shaped vanes 2 of oxygen free copper or the like are radially arranged to encircle the cathode 3. The vanes 2 have base end portions which are fixed to the inner wall of an anode cylinder 1 of oxygen free copper, or integrally formed with the anode cylinder 1. Two inner strap rings 9, which are selected to be identical in diameter to each other, are provided on upper and lower ends (in FIGS. 2A and 2C) of the vanes 2. The inner strap rings 9 are arranged in positions separated by a l prescribed distance from the forward end portions of the vanes 2 (see FIG. 2c) with respect to the full length L of the vanes 2. Further, two outer strap rings 10, which are selected to have the same diameters, being larger than those of the inner strap rings 9, are provided on the upper and lower ends of the vanes 2. The inner and outer strap rings 9 and 10 are so fixed to the vanes 2 as to short-circuit every other vane 2. In other words, the upper one of the inner strap rings 9 and the lower one of the outer strap rings 10 are fixed to the same alternately-arranged vanes 2, while the upper one of the outer strap rings 10 and the lower one of the inner strap rings 9 are fixed to the remaining vanes 2 respectively. The respective adjacent vanes 2 and the inner wall of the anode cylinder 1 define spaces 14 (see FIG. 2b)partially opened toward the cathode 3, thereby to form cavity resonators. The oscillation frequency of the magnetron 100 is determined by the resonance frequency of such cavity resonators. In a central portion of the anode cylinder 1, a cylindrical space is axially defined by the forward end portions of the vanes 2. The cathode 3 is arranged in this space. The space 4 thus held between the cathode 3 and the vanes 2 at a prescribed distance is called an interaction space. A uniform direct-current magnetic field is applied to the interaction space 4 in parallel with the central axis of the cathode 3. To this end, permanent magnets 12 are arranged in the vicinity of upper and lower ends of the anode cylinder 1 respectively (see FIG. 2a). Direct-current or low-frequency high voltage is applied between the cathode 3 and the vanes 2.
In FIG. 2c, the cathode 3 is formed by the filament 5, which is helically prepared from tungsten containing thorium or the like, a top hat 7 supporting the upper end of the filament 5 and having a flange part 6 which is larger in outer diameter than the filament 5 in its upper portion and an end hat 8 supporting the lower end of the filament 5. The top hat 7 and the end hat 8 are formed of a metal having a high melting point, such as molybdenum. The top hat 7 and the end hat 8 are adapted to prevent axial deviation of electrons from the filament 5. Alternate ones of the vanes 2 are electrically connected with each other since the inner strap rings 9 and the outer strap rings 10 are alternately fixed to the upper and lower ends of the vanes 2, as hereinabove described. An antenna conductor 11 (see FIGS. 2a, 2c) is so provided that an end thereof is connected with one of the vanes 2.
In the aforementioned structure, high-frequency electric fields formed in the cavity resonators are concentrated to the forward end portions of the respective vanes 2, and partially leak into the interaction space 4. Since the inner and outer strap rings 9 and 10 couple alternate ones of the vanes 2, the respective adjacent vanes 2 are at reverse potentials in high frequency. An electron group emitted from the cathode 2 rotates about the cathode 3 in the interaction space 4, whereby interaction takes place between the electron group and the high-frequency electric fields, to cause oscillation of microwaves. The microwaves obtained by such oscillation are outwardly guided through the antenna conductor 11 which is connected with one of the vanes 2. Since conversion efficiency into microwave power is not 100%, the energy of the electron group is partially consumed as heat. Therefore, fins 13 (see FIG. 2a) are provided along the outer circumference of the anode cylinder 1 for radiating the heat. FIG. 2B, 2C shows only the internal structure of the anode cylinder 1, and fins 13 etc. are not shown in this figure.
International Standards are established by ITU (International Telecommunication Union) for the aforementioned magnetron, and the basic frequency of 2450 MHz is allocated to food heating apparatuses, medical appliances, parts of industrial instruments and the like. In such application, therefore, the magnetron 100 ideally oscillates only microwaves at the basic frequency of 2450 MHz (.+-.50 MHz), whereas the same generates various higher harmonics in practice. Within such higher harmonics, particularly the fifth harmonic having a frequency of 12.25 GHz (.+-.0.25 GHz), there is an overlap with a working frequency range of satellite broadcasting, which has been tested since around 1981 and more recently has been in use, creates serious problems. For example, while radio frequency allocation for SHF satellite broadcasting is varied with areas of nations, the frequency range thereof is set in a range of 11.7 to 12.75 GHz.
In the magnetron having the aforementioned structure, further, the filament 5 is abnormally heated by generation of cathode back bombardment, whereby the filament 5 may be fused in an extreme case.